The present invention relates to periodic arrayed waveguide grating multiplexer/demultiplexers, particularly multiplexer/demultiplexers for separating periodic sub-bands bands of multiple channels.
Optical telecommunications systems currently utilize dense wavelength division multiplexing (DWDM) to transmit multiple optical signals at different wavelengths in order to increase the available bandwidth of the optical fiber network. In the past few years, the number of channels of DWDM systems has dramatically increased from a few channels to more than one hundred channels.
A maximum capacity system is not always required. Service providers would prefer to provide a lower cost system that can be expanded as demands require. To increase flexibility in the optical telecommunications network, service providers propose dividing the communications window into multiple channel sub-bands, which can be addressed modularly as system hardware upgrades are required.
To support a modular sub-band system, a multiplexer/demultiplexer adapted for use with sub-bands of multiple channels smaller than the whole DWDM system is required.
For practical reasons, this discussion refers to 40 channels of the optical telecommunications window at the standardized 100 GHz ITU channel spacing. It is understood, however, that this is just an application example and that a larger or smaller number of channels and different channel spacing can just as easily be accommodated.
An arrayed waveguide grating (AWG) is a dispersive optical device suitable for multiplexing and demultiplexing a large number of channels simultaneously. Channel signals on each channel of a 40 channel system can be multiplexed and demultiplexed in a single AWG.
Selecting an appropriate multiplexer/demultiplexer for a modular system of multi-channel sub-bands introduces difficulties for the service provider in hardware costs and complexity. If only a small subset of all the channels must be multiplexed/demultiplexed, then unnecessary losses and complexity are introduced by using a large device to multiplex/demultiplex the whole set of channels. Furthermore, using a large capacity AWG to pick up only a selected smaller subset of channels, requires a large switching router to couple the selected outputs. Alternatively, an AWG can be designed and optimized for any specific subset of channels. However, for the service provider to provide service at a selected sub-band of channels and later add service for additional sub-bands, it is costly and inconvenient to maintain a specific AWG for each multi-channel sub-band.
Thus, it is desired to provide a single multiplexer/demultiplexer that can multiplex/demultiplex any selected multiple channel sub-band of a plurality of sub-bands. Since the AWG is a periodic device in frequency, it can be used to address more than one sub-band, given that the sub-bands are equal to the free spectral range (FSR) of the AWG. However, this is not sufficient to design a suitable multiplexer/demultiplexer, because unacceptable losses are experienced in such a device. A better multiplexer/demultiplexer is still needed to provide a modular system of multiple channel sub-bands.
A prior art device is described in a paper, Transmission Characteristics of Arrayed Waveguide Nxc3x97N Wavelength Multiplexer by H. Takahashi et al., Journal of Lightwave Technology, Vol. 13, No. 3, March 1995. In the device described, a router is constructed having a same number (N) of input and output waveguides. The Nxc3x97N routing is achieved by using the periodicity of an AWG. By selecting different inputs, an order of output channels can be shifted. Each output has periodic pass frequencies for routing any one channel of a multiplexed signal to any output. In this device, the Nxc3x97N connection is provided when FSR=N*xcex94F, wherein xcex94f is the frequency channel spacing set at 100 GHz. An insertion loss of 3 dB is claimed.
The present invention has found that by designing a periodic dispersive element, particularly an arrayed waveguide grating with an FSR broader than the selected sub-band width by a multiple corresponding to a number of inputs greater than one, any selected multiple channel sub-band in the range of wavelengths of interest can be multiplexed or demultiplexed. And further, that by optimizing the number of inputs and outputs, losses can be significantly reduced.
Accordingly, the present invention provides an optical multiplexer/demultiplexer for multiplexing/demultiplexing any selected sub-band of M adjacent channels of optical frequencies from a plurality of sub-bands comprising:
a first planar waveguide;
a second planar waveguide;
an arrayed waveguide grating optically coupling the first and second planar waveguides having a periodic free spectral range;
a plurality of spaced apart waveguide inputs N coupled to the first planar waveguide, the N inputs for launching complementary sub-band frequencies of adjacent channels in a demultiplexer mode of operation, or for outputting a multiplexed band of adjacent channels in a multiplexing mode of operation;
a plurality of spaced apart waveguide outputs M coupled to the second planar waveguide for receiving demultiplexed output channel frequencies of an input sub-band in a demultiplexer mode of operation, or for launching a plurality of adjacent channel frequencies in a multiplexing mode of operation;
wherein the M outputs are separated to provide a spectral frequency interval xcex94f, the inputs are separated to provide complementary input sub-band frequencies each spectrally separated by substantially M*xcex94f, and the free spectral range of the device equals substantially N*M*xcex94f.
Advantageously, the arrayed waveguide grating in accordance with the present invention can provide multiplexing/demultiplexing functionality for any channel sub-band over a broad channel spectrum.
Further advantages of the present invention will be apparent to those of skill in the art from the following figures, which illustrate preferred examples of the invention by example only.